1. Field of Invention
the present invention concerns a method for making a long tubular thermoplastic item with bottom at one end.
2. Description of Related Art
Long tubular thermoplastic items, e.g. with bottom at one end, may be used for many purposes, e.g. for pneumatic cylinders, scaffolding rods, structural elements for latticework and for cleaning tools, such as broomsticks.
For cleaning tools of different kind there are handles made with different designs, e.g. ergonomically advantageous designs, and which are made of different materials, typically aluminium or wood.
Within the later years it has been more widespread to use synthetic material for making such handles. In order to save weight, handles for brooms, floor scrapers and other cleaning tools, for which is required a relatively long handle, are made as hollow handles produced by extruding thermosetting plastic material. However, using extrusion for making such handles means that the handle is made with a certain external diameter along the entire length of the handle. If such a tubular item is to be closed at one end it is common to shut off an open tube after the moulding process itself. Also, possible separate components, such as external fastening devices, mounting faces, handle facilities or suspension facilities, are retrofitted.
In order to change such a given handle profile, it is known to apply a film or a tubular stocking which is pressed on the handle as e.g. described in International patent application WO 02/34494 by Toft. Such a film or tubular stocking is typically made of a thermoplastic material, which also makes the handle more comfortable to hold on. Typically, polypropylene is chosen as this thermoplastic material.
In order to achieve satisfactory use of the handle for different tools, auxiliary means are to be fitted on the handle, e.g. enhanced non-skid surfaces, holder devices of auxiliary equipment, relief discs, or clamping devices for telescoping handles, where these separate components are mounted on the handle by means of welding, bonding or by means of rivets. However, this means that pockets and grooves appear in which dirt and bacteria may accumulate, which is a great problem for the food industry in particular, as it is of decisive importance in the food industry, e.g. slaughter houses that the tools are kept clean. Cleaning of the tools is effected by autoclave which is a cumbersome and expensive process. Also by pressed on, tubular threaded pieces for mounting tools there is great danger of accumulating dirt. It has appeared that the previously mentioned stocking described in International patent application WO 02/34494, which is pressed onto an aluminium handle, does not fulfil the requirements to light cleaning in the food industry in a satisfactory way, as an interspace remains at the transition between the end of the stocking and the handle in the order of magnitude of 0.1 mm where bacteria may accumulate and grow.
In general, however, it is known to make longer, tubular items that are closed at one end. For example, there is known a manufacturing method for a hockey stick described in International patent application WO 91/08803 by Motley. This gas injection technique, however, leads to items where the internal hole does not have a well-defined diameter. For example, such a technique cannot be used for making telescoping tubes. Similar problems are experienced with a related technique called liquid injection technique, where liquid, typically water, is injected together with the melted plastic mass.
Further techniques for moulding tubular items exist, e.g. mould blowing and spray blowing, which are used for thin-walled plastic bottles, e.g. shampoo containers or screw-capped lemonade bottles. Such a technique is described in U.S. Pat. No. 3,301,928.
In general it is also prior art to make handles with bottom at one end by injection moulding. Typically, these items are rather short due to lacking stability of the mould core in the hollow, tubular item during the mould process. Thus there may be made handles for hand tools, e.g. as described in European patent application EP 901 887 by Hoepfl or in U.S. Pat. No. 4,646,405 by Reinhold.
Long items may be injection moulded in moulds with fixed mould core. By using a fixed core during the mould process, a number of advantages are achieved. For example, it is ensured that the hold has a well-defined diameter and a well-defined positioning, also by very long items, whereby is achieved a well-defined weight distribution and weight of the item and also allowing use of the item for telescoping tubes. However, a number of problems have been recognised by injection moulding a long tubular item while using a mould core, if this mould core is not supported at both ends. One of the problems is non-uniform pressure action of the injected mass on the mould core, which makes the mould core to deflect to a side with a not centred hole as consequence. An example of how a fixed mould core is also an integrated part of the product is known from DK patent application 6426/89 by Sorensen.
In order to avoid bending of the mould core, there is required a very exact and uniform injection into the injection chamber so that a uniform force action is achieved over the whole circumference of the core. Furthermore it is an advantage that the core has very great bending strength. These two factors decide how long mould cores may be used in dependence of their cross-sectional area. In order to avoid that the mould core is deflecting to the side during injection of the plastic mass in the mould process, as described in International patent application WO 02/34494 by Toft, one has used to support the core at its free end during the injection moulding process. In order to manufacture a tube, which is closed at one end, the mould core is retracted a distance in order to release this from the support immediately before finishing the moulding. The method described in WO 02/34494 is used for making a tubular handle in another stabilising tube is inserted in order thereby to make a handle with a suitable handle. The stabilising tube is necessary as the handle itself is of too soft material to be used in long handles. The method of manufacturing described in this International patent application has appeared in practice only to work for handles with a maximum length of about 70 cm, as the mould core otherwise deflects in spite of this support at both ends. This method is therefore not suited for making long plastic items with high stability as desired with handles for cleaning tools.
Another known problem is that the plastic mass after solidifying binds to the mould core. When taking plastic items off long mould cores, it is therefore aimed at making the mould core conical so that the clamping force around the mould core of the solidified thermoplastic material causes the mould core to slide slightly up from the conical faces, thereby reducing the clamping force. Then the removal from the mould is effected, e.g. by the mould core being drawn backwards through a hole with the same cross-section as the mould core at its broadest cross-section, or that a plate with a hole of the same cross-section as the mould core at its broadest cross-section is pushed forward over the mould core and pushes the item off the mould core. Alternatively, removal from the mould by means of pressurised air is used, which is pumped through the mould core into the moulded item whereby this is released from the mould core. This technique is mentioned in the above described International patent application WO 02/34494. A pressurised valve for a mould core and a method of removal from mould is described in European patent application EP 862 980.